<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>rem</title>
</head>
<body>
    <div class="parent">
        <div class="son">

        </div>
    </div>
</body>

<style>

  html{
    font-size: 30px;
  }

  .parent{
    width: 10rem;
    background-color: aqua;
    height: 200px;
    font-size: 20px;
  }

  .son{
    width: 5em;
    background-color: blue;
    height: 200px;
  }
</style>    
</html>